Range heater and control



July 1, 1952 YOUNG 2,602,132

4 RANGE HEATER AND CONTROL Filed Dec. 24, 1949 2 SHEETS-SHEET lInventov: James F Young,

by W His Attofneg.

July 1, 1952 YOUNG 2,602,132

RANGE HEATER AND CONTROL Filed D60. 24, 1949 2 SHEETSSHEET 2 Inventor:James F. Young His Attorney.

Patented July 1, 1952 RANGE HEATER AND CONTROL James F. Young,Bridgeport, Conn., assignor to General Electric Company, a corporationof New York Application December 24, 1949, Serial No. 135,015

3 Claims.

This invention relates to automatically controlled heating devices, andmore particularly to surface heating units for cooking appliances withan automatic temperature control.

' In the process of cooking on surface heating units, particularly inconnection with electric ranges, it is often necessary to adjust theinput to the heating unit several times so that the desired uniformcooking temperature is maintained.

In every instance, it is preferable if the cooking utensil is initiallyheated quickly to the proper operating temperatures Thus, a person usingan electric range frequently turns a heating unit on to the highestavailable rate of heating to brin I the cooking vessel up to the desiredtemperature,

after which a'lower rate of heating is selected to maintain the propertemperature. However, it is found that if the operator waits until thecooking utensil reaches the desired temperature be foreswitching to alower rate of heating, there 'is often serious overshoot of the desiredtemperature causing burning of foods or smoking of fats.

This is due to the fact that the heating unit itself heats up morerapidly than the cooking utensil, and, because of its relatively highthermal mass,

'it continues to supply heat to a utensil for a period of time after itis turned off or to a lower heat. In addition, it is found that theextent of overshoot depends upon several variable factors.

For example, in boiling operations, pressure cooking, and heating ofdeep fats, little if any overshoot occurs since in each case a largeheating load is involved and the additional input after the heating unitis shut off makes relatively little diflerence in the temperature of thevessel or its contents. Alternatively, operations such as griddling andfrying which present a much smaller heat load require considerableanticipation in turning off the heating unit if serious temperatureovershoot is to be avoided. In addition, the type of pan employed has aninfluence. For example, cast iron, being a poorer conductor, needs moreanticipation thanaluminum, a good thermal conductor.

It is, therefore, a primary object of my invention to provide anautomatic temperature control for a heating unit of a cooking appliancearranged to bring any cooking utensil quickly up to a preselectedtemperature and thereafter'maintain that temperature within narrowlimits. A further object of my invention is to construct such anautomatic temperature control I which anticipates the extent ofovershoot which may occur and'thereby' shut oil the unit prior toarrival at thepreselected temperature.

oriented in a manner so that it is principally sensitive to thetemperature of a cooking utensil resting on the'heating unit. One of thefeatures of my invention consists in arranging a heat pipe to conductheat from the heating element to the thermally sensitive resistor toprovide anticipaion and prevent temperature overshoot. In accordancewith another feature of my invention,

temperature overshoot is avoided by the use of an additional pair ofthermally sensitive resistors having high temperature coefficients ofresistance with one of these resistors oriented in proximity to thecooking vessel and the other resistor more remote. Thus, where there israpid heating of the cooking utensil a relatively large differentialbetween the resistances of these two resistors occurs. This differentialin the resistances is emplayed as an anticipatory control to deenergizethe heating unit before the preselected temperature is reached. I

The features of my invention which I believe tobe novel are set forthwith particularity in the appended claims. My invention itself, however,both as to its organization and method of operation, together withfurther objects and advantages thereof, may best be understood byreference to the following description taken in connection with theaccompanying drawing in which Fig. l is a bottom plan view of a heatingdevice or unit embodying my invention; Fig. 2 is a crosssectional viewtaken along the line 22 of Fig. 1; Fig. 3'is an enlarged cross-sectionalview of a detail; Fig. 4 is a diagrammatic view of the variouscomponents and electrical connections for the automatic control; andFig. 5 is a partial crosssectional view similar to Fig. 2 of a modifiedform of my invention. I

While the present invention is particularly adapted for use inconjunction with surface heating units of anelectric range, I do notwish to be limited solely to this one application of my invention, sincethe 'controlarrangement disclosed and claimed herein may find many otherapplications in various types of heating devices and electric heatingunits. Referring to Figs. 1 and 2 of the drawing, I have shown anembodi- 3 ment of'my invention arranged to control an electric heatingunit of the type commonly employed for the surface heaters of anelectric range. While the particular form of the heating elementemployed forms no part of the present invention, preferably it comprisesa helical resistance conductorgl mounted within an outer metallic sheath2 and supported in spaced relation with reference to the outer sheath byan electrically insulating heat conducting mass 3, such as highlycompressed granulated magnesium. oxide. A completed heating element 4 isarranged in a spiral coil with the turns supported in spaced relation ona plurality of supporting arms such as 5, 6, and The innermost end ofeach of these supporting arms may include a flange or foot portion suchas 8 for rigid attachment to a cylindrical shell or heat barrier 9. Theouter end of each of the supporting arms includes an ear I!) adapted toproject through. an aperture formedina downwardly depending, portion llof an annular flanged member I2. Flanged member It? in turn is adaptedto rest on any suitable supporting surface such as the main workingsurface on the top of a range. V V

. The temperature sensitive elements, which control the, extent, ofheating of any cooking utensil placed on heating unit 4, are mountedcentrally within the heatingunitand withinthe cylindrical heat barrier9. As shown in Fig. 2, the present embodiment employs three temperaturesensitive devices l3. ,l4, and I5. Allofthese temperature sensitivedevices are carried on the underside of, a metallic disk [5, whichpreferably is highly conductive both thermally and electrically, forexample, of aluminum or copper. Disk I is resiliently supported withrespect to, the heating unit. so that its uppersurface normally extendsslightly above the upper surface of the heating element. In the presentinstance, such a resilient mounting is achieved by use of a coil springI! attached to the heat barrier 9 by a plurality of lugs 18. Spring I1is selected so that the lightest load presented by any cooking utensil Ynormally used is sufficient to depress disk I6 and permit thecookingutensil to rest directly on the top surface of heating unit 4. By use ofthis resilient mounting, it is apparent thatthe thermally sensitivedevices I3 and M are always maintained in good thermal contact with thebottom surface of, a cooking vessel. g

Placement of an object onthe heating unit in accordance with thisarrangement serves to energize heating element 4. To accomplish. this, a

fixed contact la is carried by an arm riveted or otherwise secured toheat barrieril. Movable contact 2| is carried on a resilient contact arm22 which also is supported by heat barrier 9. Screw 23 and nut 24 forattaching contact arm 22 to shields are insulated with respect to theshield by insulating washers 25 and insulating spacer 26. With thisconstruction, whenever disk I6 is in its raised position contacts I9 and2| are closed.

However, when disk I6 is depressed, a terminal conductor 21, projectingfrom heat sensitive device l3 and connected to resilient arm 22, movesdownwardly forcing contact 2| away from contact IS. The manner in whichthis action controls the energization of the heating unit appears at alater point of this disclosure.

In Fig. 3, I have shown in detail the construction of heat sensitivedevice [3. The thermally sensitive element itself is here shown asacylinder 28 supported within an outer protective metallic sheath orhousing 29. One of the prigrees-F'. for griddli grations. A thermallysensitiveresis'tor, the resistance of which changes from 500 ohms to 20ohms over this temperature range provides a very satisfactory andreliable control. employing the arrangement disclosed Furthermore, thiselement must be capable of withstanding accidental overheatingtemperatures as high as 1000 degrees F. It is also desirable, of course,that the assembled heat sensitive device be capable of withstanding thevarious mechanical shocks to which it is subjected during normal usage.In view of these various requirements, I find a thermistorparticularlysatisfactory for the thermally sensitive element 28, andhereinafter element 28 will bev referred to as a thermistor. By thisterm is meant any resistance element having a resistance which variesgreatly with changes intemperature. As shown in Fig. 3,,the thermistoris insulated on its outer cylindrical surface by an insulating sleeve30, and on its bottom surface by an insulating disk 3|. Preferably.insulation 30 and. 3| is of material such as mica, not readily affectedby the higher temperatures hereinvolved. The top surface of thethermistor is-in direct contact with metallic housing 29,- .Housing 29inturn may include a head portion; adapted to be riveted in position inan aperture 3ZProvided in disk Hi. The lower edge of housing 29- isrolled inwardly to compress insulating disk 31- and the upper portion ofterminal wire 2'! against the bottom surface of the thermiston; Theassembled device may be completely sealed with a suitable molded ceramicmaterial 33, as shownin Fig. 3., With this construction, it may be seenthat one of; the terminals forthe thermistorcomprises the outer metallichousing 29 and'disk [6, which inturn are electrically grounded; to: theframe ofthe range. The other end terminal for the thermistor is formedby thelconductor, 21. This orientation of the; thermistor, makes; itthermally sensitive to the temperature of'disk H5 which is pressedagainst thebottom of any cooking utensil placed thereon. Hence,thermistor 28, as it; is embodied. in heat. sensitivefdevice 3, isprincipally-sensitive to the temperature .of such cooking utensil. Tothis'same. end, shields; reduces transfer of heat by radiation. from theinside turns of heating element 4,-..

It should be. understood at this point that thermally sensitive devices.M: and. |.5. are, substantially identical. with thermally sensitive.device |3,,the details of constructionof.whichhave been explained inconnection with Fig. i. 3. Device M is attached to; the undersurfaceofdisk IS in a manner similargtoz the attachment of device I 3.Device I5, however, is supported on the lower endofa bracket 3.4. theupper'end of a e-:.un t,.- it h ats up r latively; slowly-Thermallysensitive ;de vic,e.=.| ln'of. course. follows quite closely;the. temperature ;of.:' the cooking utensil, and when the temperature ofthe nnnlzino' utensil rises. slowly thereis sufficient time for heat'totravel down the bracket 34 to device, l5. Hence both devic'e's l4 and.I5 remain at substantially th me temperature, and hence theirresistances'reina'in substantially the same. However,' .iffa light}thermal load is placed on the heating un it, suchasa thin aluminumskillet, its temperature rises very rapidly.-In' view of its proximityto the bottom surface of such utensil, thermally sensitive-device [4also heats up rapidly; However, because of its remote posi-' tion';there is insufiicient time for heat to travel to thermally sensitivedevice l5 and asa result its temperature rise lags considerablythetemperature ris'e or device l4. A fairly wide-temperaturedifferential-thereby exists between device l4 and device l5. The extentof this temperature, and hence-resistance, difierential depends directlyupon the rate" of heating of the cooking utensil. The resultingresistance differential is then utilized for anticipation control toprevent temperature overshoot as appears later. As described inconnection with temperature sensitive device l3,-.one of the terminalsfor each thermistor within heat sensitive devices-l4 and I5 is providedby a ground-connection to the frame of the range: The other ter minalfor device i4 is formed by the conductor 35. Likewise, conductor 36forms a second terminal for thermally sensitive device l5. 1

From the above, it may beseen that automatic temperature'control isachieved through the temperature responsive resistance change of devicel3 and the resistancedifferential between devices I4 and 15. .The mannerin which this function is accomplished becomes clear from an examination.of Fig. 4. The main heating element 4 is adapted to .be connectedacross power supply lines 31 and 38 through a manually operated doublepole single throw switch 39 and an automatically controlled switchcomprising afixe'd contact 48 and a movable contact 4|. Movablev contact4! is carried on one of the ends of a resilient contact arm 42, theother end; of which is fixedtoany suitable support. Withline switch 39closed andcontacts 40 and 4| closed, itmay be seen that full linevoltage, for example; 220 volts, is applied across .the heatingelementsothat. it heats uprapidly.

Contacts 149 and 4t long with contact arm 42 ;ma y bepositioned at anyconvenient.- point where they are not thermally aifectedby heatingelement 4..;; Contac ts 40 and 4| arenormal 1y open, but are adaptedtobeclosed by the heat responsive motionof abimetallic strip 43, thefreeend of which '5 movable between a fixed stop 44 and the outer sideof contact arm 42. Encirclin the bimetallicstrip is a small heatingwinding 45 capableof dissipatingsufiicient heat when-energized to causethebimetallic strip to defiectand close contacts' 4b and 4 l To obtainprompt responsive action of the bimetallic element, it is preferredto'suppo'rt its fixed, end on a friction loaded pivot; 1' 'As' hereshown, the fixed trallyi apfllilllimd cyundncai" member H46. The

hd of the mineral. as attached, thereto a Cen- I central aperturepermits. member 46' to be sup-J ported on a'ffixed' ing' pm: about whichthe bimetallic strip are. :ro limit this rota' t 'n, i ric tionalsurfacejv'l isjprepared'jon the atria resilient arm 48' attachediin anysuitable er 'to' a'fixedsupporting surface. This'fric tiio'nalengagement for. jsurfacefflj; with the outer bra n a1 7 strain the pivotmovement of the i bimetallic member." itends'i r 6. the initial heatresponsive motion of the bimetallic strip consists only. in-a movementof the free end in a direction to close contacts '40 and. However,oncethe contacts have been closed and the desired contact pressurereached, further heat'responsive motion of the bimetallicelement is inthe nature of a pivotal movement of its fixed end about the axis ofmember 46. Thereafter, when the heating winding 45 is no longerenergized, cooling of the bimetallic element-results in an early openingof contacts and 4! without waiting for the'bimetallic strip to return toits initial position. As the bimetallic element continues to cool, itsfree end abuts against stop 44, fol1owed eventually by a pivotalmovement of the other end of the bimetallic strip. It is therefore seenthat the addition of the friction loaded pivot materially decreases theresponsive time foropening or closing the contacts upon cooling orheating of the'bimetallic element. 1

To control the supply of power to heating winding 45, a suitable gaseoudischarge device 49, such as a thyratron, is employed. This circuit maybe traced from a neutral 50 of the power supply through the platecircuit of the thyratron,

L heater-45, tothe line 38. Thus, whenever the thyratron is conducting,heating winding is energized to deflect the bimetallic strip and closecontacts and'4|. The gaseous discharg device 49 may be of a convenionaltype including acathode5l, cathode heater 52, anode 53, shield grid 54,and controlgrid 55. Acapacitor 56 may be connected across lines 38 and59 to suppress radio interference.

The control arrangement of the present embodiment is adapted for usewith thermally sensitive device l3, in which the thermistor is onehaving a'high negative temperature coefficient of resistance. Obviously,this control could be arranged to operate in response to changes inresistance of a thermistorhaving a high positive temperature coefficientof resistance. It may also be noted that thermally sensitive devices I4and ['5 include thermistors, both of which have either high positive orhigh negative temperature coefiicients of resistance. I 1

Conduction through the gaseous discharge device 49 depends upon themagnitude and phase of the potential applied to'control grid 55. Inaccordance with this invention, the control grid issubjectto a fixedgrid bias, and in addition is influenced by the relative resistance ofcontrol device-l3 was well as the resistancediflerential between controldevices l4 and [5. Referring to Fig. 4, the various control voltagesrequired are provided through the use of a transformer having itsprimary '51 across power supply lines 38 and with three secondaries 58,59, and 60. Secondary 58 along with a fixed resistor GI- and capacitor52 form thefixed grid bias. Secondary 59" of -this' transformer iscentertapped for 'con nection in abridge circuit with thermallysensitive device I3 and a variable resistor 63. Resistor'63 may beadjusted over a range ofresist- 'ance' values of substantially the" sameextent as the range of resistance changes occurring in temperaturesensitive device [3 during use. The outpu' fromthis' bridge is connectedacross the pri n arjy=' 6 41"of a second transformer, the secon ary '55of which connected in a manner to when 't in aer i t m d;

' point thattlie'outputvoltageoitransformer drj relativelylow value; rpri ex .v lisf to tiie frame of the rangeandlikewi'se grounding of one ofthe terminals of the thermistor included in temperature sensitive device[3, as vexplained in connection withFigs; I, 2', and3. This arrangementalso permits use of asinglecontrol conductor for connection throughcontact arm 22 to temperature sensitive device l'3'. Also, as explainedin connection-with Fig. 2 fixed contact I9 is grounded to thejrame ofthe range through its mounting bracket and heat barrier 9.- Referringagain to Fig. l, it may bo-seen that whenever contacts is and 2t areclosed corresponding to the conditions existing when there is no cookingutensil on the heating unit, both terminals of the heat sensitive deviceI3 are at groundpotential and, hence this element appears in the circuitas having averylow'resistance, ;However, when contacts l9 and 2| areseparatedin response to the placing of a vessel on the-heap ing unit,the normal resistance of device I3 is eifective asanelement-of the bridgcircuit. In view of the low voltage employed inthis bridge circuit,preferably the transformer comprising primary 64 and secondary 65 servesto amplify bridge output in the control grid biasing circuit.Transformer secondary 60,- as shown in Fig-. 4, is also centertapped andconnected in abridge circuit with, the resistance elementsof thermallysensitivedevices- I4 and I5, ihe output of this second bridge circuit isimpressed across the terminals of a transformer having aprimary '66 andsecondary 61. This secondary 6'! along with secondarie 65 and 58comprise the potentials connected in series with resistor 6| andcapacitor 62 to determine the effective controlgrid bias. Transformersecondary; 60 also has a relatively low voltage output permittinggrounding of. one terminal each of the thermally sensitive devices I4and I5, asillustrated and explained in connection with Fig. 2.. Theother terminals of these devices Hi and [5 are connected respective-' lyto the terminals oi transformer secondary; Also, the output of; thissecond bridge circuit may be amplified to varythe grid ,bias through thetransformer comprising primary 6% and secondary 6-1. v r v Assumingthatcontacts l-9= and 21: are openin the first bridge circuit, when;theimpedance of adjustable resistor 63 and the impedance of-temperature-sensitive device l3= are identical, the bridge ispbala-nce'dand nopotential is applied across the terminals of. transformer primary.64-.Likewise, in, the second: bridge circuit, when the resistancesthroughtempferature sensitive devices l4 and I5 are identical thebridgeis balanced and no potential is appliedacrossthe terminals oftransformer primary 66.- Under these conditions, the circuit componentsmaking up the fixed grid bias are selected se that the control grid 550f thyratron 49 is at th critical cutoif value, and hence no conductiono-ccurs through the thy-rar is re rs means at he t aw din 45 isineffective tooperate the bimetallic strip andfclo'se contacts 40 and IIinthe main heater circuit. Howeveijshould the resistance throughthermally sensitive-device I 3' rise'diie to a decr'easei'ntennneiature,- the first-bridge becomes unbalanced resultingv in; aphase and magnitude shift of the ccsntraigndtiasmg Voltage in a mamiiefftol cause amass I49 tbicfoiiduct and hence erect-a closure ofcontact JJBIJa'ndJ-IIf Furthermore, it be seen that if ia," cookingutensil is r''indid from the heating un permitting onthefirs't' bridgeas a very mzntemperature or devicel3. Under these conditions; the firstbridge is, unbalanced in the opposite direction to bias the controlgr-id of thev thyratron well beyond its critical cutoftvalue to. preventconductionthrough the thyratron and-heating windings.- Hence, wheneverthere is no weight on the heating unit effective to separate contactsl9'and 2|, heating unit remains deenergized. Adjustable resistor 63maybe varied in any suitable manner, for example by a -control knoblocated ate. corivenient point with respect to therange. Preferablythevar-iable 0r adjustment member is calibrated in degrees F. or incooking operationsto enable an one fator topreselectl the desiredtemperature to which a cooking utensil is'to be heated; ,To energizeheating unit 4 after the preselected temperaturehas been set and lineswitch 39 closed,v it is merely necessary to place the cooking utensilon the heating unit and thus open contacts [9 and 2h Initially, ofcourse, the resistance of device I3 is relatively high since it has beenassumedthat it has a negative temperature-coeffl-cient. Therefore, thefirst bridge is: initially unbalanced in a direction to supply acontrol" grid voltage effective to fire aseous discharge device 49; Theresultant heating effect through heater 45' deflects bimetallic strip43, vclosin'gconta'cts HJ- and M- to-energize heating element 4; As thetemperature of device l3' rises} its resistance decreases untilthe'point is reached Where the first bridge becomes bal anced at whichtime furtherconduction through the thyiatron is prevented by the fixedbias on the control grid; A cycling action thereafter takesplacentomaintain thecooking utensil at a: substantially uniform temperature;Anticipation controltoprevent temperature overshoot-is provided by thesecond' bridge which includes the temperature" sensitive devices M- andt5.- so long as the temperature or both devices Mand IEi's the same,their resistancesare equal andhence this second bridge is balaiicedi Howvei',- if a temperature difiel''ritial exists between devices 14 and15-, actrre sponding resistance" airrerentiai-acetate-imbalance thisBridge.- Asx plained above-, if a cooking iitens'il'is heated uprapidly, device l4 'closly renews-the temperature rise of suchutensil,but device [5, due t6 its remote; location; rises in temperatiii'e moreslowly. The extent of the temperature di'lfeffitial and henceresi'stance diiierential' depends directly upon the rate of heating. As'may beseen irom Fig.4; the resulting unbalance of the second bridge duetosuc'h a resistance differential is effective through transformerprimary 66 and its secondary G1 to bias' control grid 55 to" its cutefi'talus prior I to arrival; at the preselected temperature asdeterr'ninedby the first" bridge and temperature sensitive device l3, It may benoted" that this anticipation control provided by the-second bridgeiscreative pain-1y during the initial he ing arter es. Thereafter,devices fl'tf'a'nd I 5 approach the same temperature and are thenno'longereffective; Thereafter, t rn pasture "control is. principallyunder the influenter: temperature sensitive device .13. h

In 5" of theidrawing I have shown a iiiodi f61mOfimticipatiofi'contiolto: prevent team closiffe df and il ithermallysensitive device[sienna-dreadediiaviiigihe same effect Ma'n'yoi'tlie elements are in"the previously described tame-nee," I-

utilized the same-nu b s hols to' desigliate tli same components;.ilt'iis 1st; tome fcnaersteodnist the wiring diagram (Fig. '4)andfiiaiii'ir'of opertrol.

ation forthis embodiment are identical with the exception that rrsssrersecqnaary 6 D, there mally sensitive devices'il'aridfi, andthetransformer comprising primary 6'61 and secondary 61 are eliminated.Referring to Fig. 5, Ifhave'sho wn a partial cross-sectional viewsimilar-to Fig.1? with the temperature controlling" elements orientedcentrally within a heating unit of the same type'as described above.Likewise, a temperature sensitive device i3, of theiorm shown by Fig. 3is carried by a thermallyconductive disk l6 resiliently'moun'ted" on acoil spring l1; Again, the thermally sensitive deviceis'shielded'fromheat radiated by-the heating element by Way of a cylindrical heatbarrier 9. ,In addition, I have provided fixed and movable contacts l9and fibrespectively, which arexmounted, and which innotion in a manneridentical withjthe previouslyde scribed arrangement. j j I However,in'the embo'dim nt shown by. Fig.,5, I have eliminatedtemperaturefsensitivedevices l4. and which provided the anticipationcon- In lieu thereof, a heat'pipe'fifl is employed, with one of its endssecured, as". by welding, to a convenient point onheating'fielementflland its other end secured to the outer metalliccasing 29 of thermally sensitive device l3. Heat pipe 68 maybe in theform of a flexible cable of a highly conductive materialjsuch asicopperor aluminum, so that it conveys. heat from the heating element to thethermally sensitive device tending normally to heat it up above thetemperature which it would reach if influenced by the temperature of thecooking vessel alone.

It is found in operation of electric heating units of this general typethat the heating unit rises in temperature much more rapidly than acooking utensil resting on the heating unit. Furthermore, the heatingelement reaches a much higher ultimate temperature after continuousoperation. By the use of this heat pipe additional heat is supplied todevice [3 tending to raise its temperature. However, if the cookingvessel being heated presents a large thermal mass, necessarily resultingin slow temperature rise, most of the heat conducted through the heatpipe is conducted on through disk It to the cooking vessel because ofits lower temperature. When a lighter thermal mass is presented by thecooking utensil and its contents, the cooking utensil rises intemperature much more rapidly. However, even under these circumstancesthe outer sheath of the heating element rises to an even highertemperature, whereby heat is conducted through the heat pipe to thetemperature sensitive device. But in this instance, Where it is highlydesirable to prevent temperature overshoot, the higher temperature ofthe utensil reduces the rate of conduction from device I3 through disk[6. Therefore, with this relatively light load on the heating unit, heatflow from the heat pipe to the temperature sensitive device results in arelatively large temperature differential of device I3 above utensiltemperature. The control, as described above in connection with Fig. 4,thus turns off the heat relatively far below the control pointpreselected whereby the coasting effect due to the residual heat storedin the heating unit brings such utensil up to the preselectedtemperature.

From the above disclosure, it may be seen that I have provided animproved automatic temperature control which not only maintains acooking vessel at a uniform preselected temby reference to 10 perature,but which also anticipates arrival of a cooking ves'selat thistemperature "to allow for the additional heating'efiectoccurring afterthe heating unit is turned oii. This 1 construc tion thereby permits useof aheaiting' element.

with a high rate of heatingto bringj'the pan or other cooking vessel"quicklytd'the desired cooking temperatures "Regardless "or the; typeutensil used or'the'extent of the thermal mass presented, temperature is'raiseddu'ring the initial cycle from room temperature tothat-preselected" temperature vvithoutdanger of overshooting ;thistemperatureor the experiencing of time consuming delays-by .failure -toreach the preselected temperature on the ifiI'Sl', cycle of operation.It 'furthermay 'be' seen that the controlling elements, whicharesubjected to-the intense heateoi the heating *unit, involve no movingparts 'which might become inoperative due-tosuch temperatures or=from'fouling by food substances during-cooking. In'iact, such movingparts as are involved in the control may be located at remote pointswithin the range or elsewhere" wherethey may be adequately"protectedfrom heat and foreign substances-tom sure their reliability and longoperating life.

While the presentinvention has been described particular" embodimentsthereof, it is to be understood that numerous modifications maybe madeby those skilled in; the art without actually departing; fromfthe'invention. I, therefore, aim-in the appended claims to cover all suchequivalent variations as come within the true spirit and scope of theforegoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

'1. In a cooking appliance, an electric heating unit comprising aplurality of spaced apart spiral turns, the innermost turn defining acentral opening, a disk positioned in said opening for engagement withthe bottom of a cooking vessel placed on the heating unit, said diskbeing formed from a material of high heat conductivity, a firstthermistor mechanically connected to said disk in direct intimatecontact therewith, a second thermistor mechanically connected to saiddisk in direct intimate contact therewith, a third thermistor, a bracketof heat conducting material mechanically connecting said thirdthermistor to said disk in spaced relation thereto and through whichheat is conducted from said disk to said third thermistor, said bracketdefining a heat path of restricted flow between said disk and said thirdthermistor, an electric switch controlling flow of current to saidheating unit, and electric circuit means responsive to change in theresistance of said first thermistor and to changes in the difference inresistance between said second and third thermistors for effectingactuation of said electric switch.

2. In a cooking appliance, an electric heating unit comprising aplurality of spaced apart spiral turns, the innermost turn defining acentral opening, a disk positioned in said opening for engage ment withthe bottom of a cooking vessel placed on the heating unit, said diskbeing formed from a material of high heat conductivity, a firstthermistor mechanically connected to said disk in direct intimatecontact therewith, a second thermistor mechanically connected to saiddisk in direct intimate contact therewith, a third thermistor, a bracketof heat conducting material mechanically connecting said thirdthermistor to said disk in spaced relation thereto and through combinedoutput of both said bridge circuits for effecting actuation ofsaidelectric switch.

3. In a cooking appliance, an electric heating unit comprising aplurality of spaced apart spiral turns, the innermost turn defining acentral opening, a disk Positionedin said opening for engagementqwith'the bottom of a cookingvessel placed on the heating unit, said diskbeing formed from a material of high heat conductivity, a firstthermistor mechanically connected to said disk in direct intimatecontact therewith, a second thermister mechanically connected to saiddisk in direct intimate contact therewith, a third thermistor, a bracketof heat conducting material I mechanically connecting said thirdthermistor to said disk in spaced relation thereto and through whichheat is conducted from said disk to said third thermistor, said bracketdefining a heat path ofrestricted flow between said disk and thirdthermistor, -a heat barrier positioned between the heating unit turnsand said thermistors between; said second and third ,efiecting actuationof said electric switch;

jtoishield them from the heating unit. an electric switch controllingnow of current to said heating unit, "and electric: circuit meanresponsive to chan es in the resistance-of said first thermistor andto-changes inthe difference in resistance thermistors .for

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